A ground detector brush may be used to electrically engage a surface so that current on the surface is transferred to the ground detector brush or to supply a ground where the brush is at ground potential. A ground detector brush usually consists of a brush soldered to a metal conductor. The brush of prior art ground detector brushes is used to electrically contact a surface and transfer any current on the surface to the metal conductor or supply a ground where the metal conductor is at ground potential. Thus, the metal conductor of prior art ground detector brushes is used to transfer current from the brush along its length to another current transferring device attached to the metal conductor or to supply a ground where the metal conductor is at ground potential. In addition, in the prior art, the metal conductor is used to provide forces to electrically engage the brush against the surface.
In order for the metal conductor to provide forces to electrically engage the brush against a surface, the metal conductor is normally pre-bent when formed so that when no outside forces are applied against the metal conductor (know as free state), the metal conductor will extent at an angle (in some embodiments, 7 degrees) past its centerline. As a consequence, when the metal conductor is bent back past its centerline during an active state (in some applications, 8.6 degrees past centerline for a total travel of 15.6 degrees (7+8.6), i.e., when pressed against a surface so that it extends 8.6 degrees beyond its centerline, the metal conductor will generate sufficient forces so that an electrical connection is formed between the brush and the surface.
In applications, the surface current may only be measured intermittently or a ground supplied only intermittently so that the brush does not excessively wear, necessitating frequent replacements. When the ground brush is in an inactive state of operation, i.e., when the surface current is not being measured or a ground is not being supplied, the metal conductor is withdrawn away from the surface, i.e., away from its active state position 8.6 degrees pass centerline and also away from its pre-bent or free state position. When in the inactive state, the metal spring extends at an angle (in one embodiment 10.3 degrees, for a total travel of 17.3 degrees from its pre-bent position) from its centerline and opposite in direction from the angle it extends when in its pre-bent or free state position. As a consequence, the angle between the pre-bent or free state position of the metal conductor and its inactive state position is equal to the sum of the two above mentioned angles (in one embodiment 17.3 degrees, i.e,. 10.3+7.0 degrees).
As a consequence, the metal conductor is always under strain because when in its active state, it extends at a first angle (15.6 degrees in one embodiment) past its free, pre-bent, or non-straining position and when in its inactive state its extends at a larger angle (17.3 degrees in one embodiment) past its free state, pre-bent or non-straining position. As a consequence, the metal conductor may be subject to excessive metal fatigue so that the ground detector brush may fail to produce sufficient electrical contact with a surface after repeated use.
In practice, prior art ground detector brushes are have been used in ground detector systems within exciter systems of turbine-generators to determine whether the exciter systems is improperly increasing current level due to the presence of two grounds in the exciter system. The exciter system of a turbine-generator is used to provide a current to electromagnetic field windings of the generator. In particular, the exciter system is designed to keep the generator rotor of the exciter system at a specified excitation level.
In such systems, several ground detector brushes are used in the ground detector system. A first ground detector brush is used to supply a ground where the metal conductor is connected to a ground potential. The second ground detector brush is used to provide an electrical path to monitor the exciter system's current level. The metal conductor of this brush is insulated from the ground potential. Both metal conductors of the first and second ground detector brushes are connected to voltage regulator equipment.
The voltage regulator equipment is used to monitor the excitation current level. If the excitation current level increases, the increase indicates that a second ground exists in the rotating electrical system. The increased current flow may feed both grounds causing potential catastrophic electrical arcing and melting damage. The grounds may occur anywhere in the rotating electrical system, such as in the AC exciter, rectifier, and generator rotor. If the voltage regulator equipment detects the excitation current level has increased, the ground detector brushes are disengaged and the excitation current level is not permitted to be increased further. When the two ground detector brushes are disengaged from the exciter system, i.e., no longer electrically contacting the system, at least one ground is removed since one of the brushes is held at ground potential.
Thus, it is important that the ground brushes provide adequate force to engage the exciter system so that the one brush may supply a ground and the other brush may provide a path for the excitation current. The ground detector brushes of the prior art may fail over time to provide adequate contact due to the significant pre-bent required in the metal conductors of the ground detector brushes.